Nanopore can be used to detect and characterize biomolecules such as DNA, RNA and peptide at single molecule level. Nanopore-based single molecule genome sequencing technology needs neither fluorescent marker nor polymerase chain reaction (PCR), rather it directly and rapidly reads base sequence in a DNA strand. Such sequencing technology is expected to greatly reduce the sequencing cost to achieve personalized medicine. There are mainly three manners to read DNA sequences based on nanopore techniques, including strand-sequencing by using ionic current blockade, strand-sequencing by using transverse electron currents, and strand-sequencing by using synthetic DNA and optical readout. Because of different structures and sizes of the four kinds of DNA nucleotides, i.e., adenine (A), thymine (T), cytosine (C) and guanine (G), they generate different degree of blockades to ionic current flowing through a nanopore as the DNA bases translocate the nanopore, thus resulting in different drops in the ionic current flowing through the nanopore. The ionic current blockade phenomena can be used to determine the DNA sequencing. However, the depths of nanopore in the commonly used membranes made of such as SiO2, SiNx or Al2O3 are generally greater than 10 nm, which are significantly larger than the spacing between two adjacent bases in a single-stranded DNA (about 0.3-0.7 nm). In other words, about 15 bases can pass through the nanopore at the same time, and thus it cannot meet the single-base resolution requirement for genome sequencing. Consequently, in order to achieve the single-base resolution, a functional element with size or thickness comparable to the spacing between two adjacent DNA nucleotides that enables the detection of nucleotides one at a time in a single-stranded DNA is needed.
Most recently, nanopore membranes made of mono- or multi-layer graphene (monolayer graphene has a thickness of 0.335 nm) as the functional element in the nanopore-based ionic current blockage sensors have been reported [Small, 5, 2638 (2009); Nano Letters, 10, 2915 (2010); Nature, 467,190 (2010)]. However, due to the honeycomb structure of graphene, nanopore sensors made from graphene membranes suffer from large leakage currents, thus resulting in a low signal-to-noise ratio. Furthermore, due to the strong π-π interaction, chemicals such as DNA and protein molecules in the electrolyte solution could be easily adsorbed on the graphene surface, thus affecting the sequencing detection.